Modified on-off control

ABSTRACT

A modified on-off controller is formed by adding an alternating signal to the error signal. The frequency of the added signal is sufficiently high to prevent the control element from responding at this frequency. A derivative signal can also be added.

[451 July 11,1972

United States Patent Lane et a1.

[56] References Cited UNITED STATES PATENTS 3,566,282 2/1971 Lauher et3,492,471

[54] MODIFIED ON-OFF CONTROL [72] Inventors: Donald W. Lane; Troy J.Pemberton, both of Bartlesville, Okla. .307/229 X 73 Assignee: PhillipsPetroeum Company l/1970 Crowell ABSTRACT A modified on-off controller isformed by adding an alternata m F S om m n m 1m .1 '8 mm i M w Hf mm 1 mm m P$A 0 7 9 1 n 3 m h a .0 N M W F A n H u n 539/ ing signal to theerror signal. The frequency of the added al' suflic' ntl h' to r vetthecontr 1 1 tfr .330/1 A; 328/1, 127; 307/229, y p e n e Int. [58]Field of Search responding at this frequency. A derivative signal canalso be added.

6 Claims, 2 Drawing Figures OSCILLATOR vvvv MODIFIED ON-OFF CONTROL Inthe process control field there are two basic types of control systems.One is commonly referred to as on-off" control and the other asproportionaP control. On-off control, as the name implies, refers to asystem in which the control element is in one of two positions,depending on the magnitude of the measured variable. For example, acontrol valve can be either open or closed. In a proportional controlsystem, the position of the control element is adjusted in response tothe magnitude of the measured variable. For example, the degree ofopening of a valve can be adjusted in response to the measured variable.On-off control systems are usually less expensive to install thanproportional control systems and are quite effective in certain types ofoperations. Proportional control systems, on the other hand, usuallyhave greater versatility and can incorporate such features as integraland derivative control modes.

In accordance with this invention, an improved control system isprovided which employs certain desirable characteristics of both on-offand proportional control systems. This system basically provides on-offcontrol, except that the action approximates proportional control whenthe measured variable does not differ appreciably from the set pointvalue. In the control system of this invention, the output signal froman oscillator is combined with the controller error signal to provide anoutput signal. The frequency of the oscillator is sufficiently high thatthe controlled element is unable to respond to variations of theoscillator signal. However, this element serves as a low-pass filter sothat effective proportional control is provided over certain operatingranges. This control system of this invention can also incorporate aderivative form of control which is desirable in certain operations.

In the accompanying drawing,

FIG. 1 is a schematic representation of a heat exchange system havingthe controller of this invention incorporated therein.

FIG. 2 is a schematic circuit drawing of an embodiment of the controllerof this invention.

Referring now to the drawing in detail and to FIG. 1 in particular,there is shown a conventional heat exchanger 10. A stream of water to beheated is introduced through a conduit 11a, and the heated water isremoved through a conduit 11b. Heat is supplied by the condensation ofsteam in heat exchanger 10. To this end, steam is introduced through aconduit 12a, and the condensate is removed through a conduit 12b. Atemperature sensing element 13 is associated with conduit 11b to measurethe temperature of the heated water and provide a signal representativethereof. This signal is applied to the input of a controller 14 whichreceives a set point signal I that is representative of the desiredtemperature of the heated water. An output signal from controller 14adjusts a valve 16 in conduit 12b to control the rate of condensatewithdrawal and thereby the temperature of the heated water.

Controller I4 is illustrated in detail in FIG. 2. Temperature sensingelement 13 of FIG. 1 provides an output electrical signal, the amplitudeof which is representative of the measured temperature. This signal isapplied between input terminals 13a and 13b of FIG. 2. A resistor 20 isconnected between terminals 13a and 13b. Resistors 21 and 22 areconnected between terminals 13a and 13b, respectively, and the first andsecond inputs of an amplifier 23. A resistor 24 is connected between thesecond input terminal of amplifier 23 and ground. A feedback resistor 25is connected between the output and the first input terminal ofamplifier 23. The output of amplifier 23 is connected by an inputresistor 26 to the first input terminal of an amplifier 27, the secondinput terminal of which is connected to ground. The signal applied toamplifier 27 through resistor 26 is representative of the measuredtemperature of the water flowing through conduit 11b.

Set point signal 15 is established by a potentiometer 29 which isconnected between a reference potential terminal 30 and ground. Thecontactor of potentiometer 29 is connected to the first input terminalof amplifier 27 by an input resistor 31. Amplifier 27 is provided with afeedback resistor 28. Am-

plifier 27 thus functions as a summing amplifier to compare the measuredtemperature with the set point value. The polarities of the signalsbeing compared are adjusted so that the output signal from amplifier 27is representative of any difference between the desired set point signaland the signal representative of the actual measured temperature. Thisoutput signal is applied through a resistor 32 to the first inputterminal of a summing amplifier 33.

The output terminals of an oscillator 34 are connected across apotentiometer 35. The contactor of potentiometer 35 is connected by aresistor 36 to the first input terminal of amplifier 33. Thus, analternating current signal is combined with the error signal fromamplifier 27.

A third signal is applied to the input of amplifier 33 from a derivativenetwork 37. The output of amplifier 23 is connected by a resistor 40 anda capacitor 41 to the first input terminal of an amplifier 42. Thesecond input terminal of amplifier 42 is connected to ground. Acapacitor 43 and a resistor 44 are connected in parallel with oneanother between the output and the first input terminal of amplifier 42.Amplifier 42 and its associated circuit elements thus provide aderivative network. A potentiometer 45 is connected between the outputof amplifier 42 and ground. The contactor of potentiometer 45 isconnected by a resistor 46 to the first input terminal of amplifier 33.

A Zener diode 47 is connected between the output of amplifier 33 and thefirst input terminal. A resistor 48 is connected between this firstinput terminal and ground. Amplifier 33 functions as an on-off device sothat the output signal is at one of two potential levels, depending onthe amplitude of the input signal to the amplifier.

The output of amplifier 33 is connected by a resistor 49 to the firstinput terminal of an amplifier 50, the second input terminal of which isconnected to ground. The first input terminal of amplifier 50 isconnected by a resistor 52 to a terminal 53 which is maintained at areference potential to provide a bias signal to the amplifier. Theoutput of amplifier 50 is connected to the base of a transistor 54. Thecollector of the transistor is connected to a terminal 55 which ismaintained at a positive potential. The emitter of transistor 54 isconnected by a resistor 56 to the first terminal of a coil 16a which canrepresent a solenoid employed to actuate valve 16, for exampie. Thesecond terminal of coil 16a is connected to the junction betweenresistors 57 and 58. The second terminal of resistor 57 is connected tothe first input terminal of amplifier 50. The second terminal ofresistor 58 is connected to ground. Amplifier 50 and transistor 54 serveto convert the output signal from amplifier 33 into a current signal ofsufficient magnitude to actuate valve 16. Coil 16a serves to filter theoutput current signal.

The frequency of oscillator 34 is selected to be sufficiently high thatvalve 16 is not capable of operating at the oscillator frequency. If themeasured temperature is equal to the set point value 15, the only signalapplied to the input of amplifier 33 is the output signal fromoscillator 34. This signal does not affect the position of valve 16. Ifthe measured temperature signal differs from theset point signal, a DCsignal is applied to the input of amplifier 33 through resistor 32. Theamplitude of this signal may be sufficient to cause valve 16 to beactuated. When the measured signal changes, a signal is applied throughderivative network 37 to the input of amplifier 33. The filtering actionprovided by coil 16a and the inertia of the associated mechanicalcomponents of the valve permit the controller to operate essentially asa proportional controller if the measured temperature signalapproximates the set point value. If the measured signal differsappreciably from the set point value, the control action is essentiallyon-off. While a solenoid valve has been illustrated, coil 16a canrepresent a conventional control element, such as a pneumatic controllerwhich establishes an output pneumatic signal to regulate a valve.

The particular wave form of the signal from oscillator 34 can differ forspecific applications of this invention. In some operations, asinusoidal or modified sinusoidal signal gives a desirable control,while in other operations a triangular signal may be preferred. In anyevent the frequency of oscillator 33 should be sufficiently high thatthe control element does not respond at this frequency. A frequency inthe range of 100 to 1,000 cycles per second is usually satisfactory.Derivative network 37 is not required in all operations, but isgenerally desirable because it provides a smoother control. It appearsthat the controller of this invention is primarily useful inapplications which do not have appreciable dead time or which do nothave significant lags higher than second order.

While this invention has been described in conjunction with presentlypreferred embodiments, it should be evident that it is not limitedthereto.

What is claimed is:

1. Control apparatus comprising:

signal comparing means adapted to receive an input measurement signaland a set point signal and establish a first signal representative ofany difference therebetween; signal summing means;

an oscillator adapted to provide an alternating signal; and

means applying said first signal and said alternating signalsimultaneously to said summing means, the output of said summing meanscomprising the output control signal of said apparatus.

2. The apparatus of claim 1, further comprising circuit means adapted toestablish an output signal representative of the derivative of an inputsignal applied thereto; means to apply the input measurement signal tothe input of said circuit means; and means to apply the output signal ofsaid circuit means to said summing means.

3. The apparatus of claim 1 wherein said summing means comprises asumming amplifier having a zener diode connected in a feedback pathbetween the output and the input of said summing amplifier.

4. The apparatus of claim 1, further comprising means responsive to theoutput signal from said summing means to convert such output signal intoa current proportional to the amplitude of such output signal.

5. The apparatus of claim 4, further comprising an impedance elementconnected to said means to establish a current so that said currentflows through said impedance ele ment.

6. The apparatus of claim 1 wherein the frequency of said alternatingsignal is in the range of to 1,000 cycles per second.

1. Control apparatus comprising: signal comparing means adapted toreceive an input measurement signal and a set point signal and establisha first signal representative of any difference therebetween; signalsumming means; an oscillator adapted to provide an alternating signal;and means applying said first signal and said alternating signalsimultaneously to said summing means, the output of said summing meanscomprising the output control signal of said apparatus.
 2. The apparatusof claim 1, further comprising circuit means adapted to establish anoutput signal representative of the derivative of an input signalapplied thereto; means to apply the input measurement signal to theinput of said circuit means; and means to apply the output signal ofsaid circuit means to said summing means.
 3. The apparatus of claim 1wherein said summing means comprises a summing amplifier having a zenerdiode connected in a feedback path between the output and the input ofsaid summing amplifier.
 4. The apparatus of claim 1, further comprisingmeans responsive to the output signal from said summing means to convertsuch output signal into a current proportional to the amplitude of suchoutput signal.
 5. The apparatus of claim 4, further comprising animpedance element connected to said means to establish a current so thatsaid current flows through said impedance element.
 6. The apparatus ofclaim 1 wherein the frequency of said alternating signal is in the rangeof 100 to 1,000 cycles per second.